“Now we are down here we may as well do a little exploring,” whispered Pepper. “But let us keep close together.” “Take care that you don’t fall into another hole!” answered his chum. “Wonder if we can’t find sticks, or stones, or something.” They felt around but nothing but the bare walls and bare floor met their touch. At a distance they heard a strange rumble, but could not make out what it was or where it came from. They advanced with caution and passed through the open door. All was as dark as before, but they heard a rushing close at hand. “Is that wind?” asked Andy. “No, that must be the river, or the falls, Andy. We can’t be very far from the water.” The boys had reached a flooring of wood. They passed along on this for several feet. They were side by side, trying to pierce the darkness, which was intense. A foul, musty odor greeted them. “I’ll be glad to see daylight again,” said Andy. “Do you think——” He got no further, for just then the flooring upon which they stood tilted downward. Both tried to save themselves from another fall, but in vain. Down they rolled over the flooring and fell with a splash into the water. Pepper went down several feet and his hand struck some rocks. Andy followed, and the chums grabbed each other by the shoulder. The ducking came so quickly that each swallowed considerable water. “Hi, what’s this?” spluttered Pepper, when he could speak. “We are in the river!” answered Andy. “Look out that you don’t hit your head on something!” The water was flowing swiftly, and forward shot the two boys, in darkness that was as intense as before. They felt the passageway narrowing and felt the sharp rocks on either side of them. “We can’t be in the river!” gasped Pepper. “We must be in some underground stream, an off-shoot from the river!” “I guess you are right!” groaned his chum. “But if it is an off-shoot where is it taking us?” “I don’t know.” There was little opportunity to say more, for the water was now boiling and foaming all around them. The boys were jerked first one way and then another, and all but had the breath knocked from their bodies. Andy was hit on the head by a projecting rock and dazed. “Save me!” he moaned, and Pepper clutched him and held fast. Then Pepper was struck and for several minutes knew no more. When the two boys were able to realize what had happened they found themselves still in the dark, but almost out of the water, which was rushing madly beside them. On all sides were rough rocks, some of them of immense size. From far overhead came a glimmer of light. “Are you all right, Andy?” asked Pepper, when he felt strong enough to speak. “I—I guess so,” was the gasped-out answer. “But say, I don’t want to go through anything like that again!” “Nor I! We came close to being drowned!” “Yes, and close to having our heads pounded to a jelly!” “Where do you suppose we are?”